Structural Tuning And Photocatalytic Activities Of ?-?-? Multinary Sulfide Quantum Dots Based Photocatalysts

Photocatalytic technology is directly converting solar energy into chemical energy method using semiconductor materials,owning to its high efficiency,low cost,environmental characteristics and other advantages,which is considered to be one of the most effective ways to solve the globle energy crisis.Most of the broad bandgap photocatalysts studied can only respond to ultraviolet light(only 5% of the solar spectrum).To broaden the light absorption range and improve the utilization light energy,visible-light-active multinary sulfide photocatalysts have become a research hotspot.The advantages of quantum dots possess unique optical properties and particles of a few nanometers level.The quantum confinement effect makes its size-dependant optical properties could be adjusted,but there still exist some critical issures such as the complicated preparation method and the difficulty to maintain the excellent performances in large-scale application.In this thesis,focusing on the central issues of reduction the quantum dots defects,enhancing the photogenerated carrier migration rate and effectively suppressing the recombination of electrons/holes,a series of lowtemperature aqueous phases preparation and post-treatment methods were developed for Ag-In-Zn-S quantum dots-based photocatalyst,which realized the gram-scale controllable preparation of high quality ?-?-? quantum dots and quantum dot heterojunctions.The mechanism of photocatalytic degradation and hydrogen production of the systems was explored from the aspects of component regulation,such as chemical composition regulation,surface coating,heterojunction construction and co-catalyst loading.It optimizes the performance and reaction conditions of quantum dots,which also lay the foundation for realizing industrial amplification applicatio ns.The main contents of this paper are as follows:1.The Ag doped ZnIn2S4 quantum dots were synthesized by hydrothermal method.The effects of Ag dopant on the crystal inity,microstructure,optical properties,chemical composition,optical absorption and photocatalytic decomposition of Ag:ZnIn2S4 quantum dots were studied.The experimental results show that there is the volcanic type between the hydrogen production efficiency and the Ag content.The reason for the enhanced catalytic activity is mainly ascribed to the broadened visible light response range,reduced internal defects,improved separation efficiency of photogenerated electrons/holes.However,high concentration of Ag will become the recombination centers for the photogenerated carriers,resulting in decreased photocatalytic efficiency.The optimized hydrogen production rate of Pt-loaded Ag:ZnIn2S4 quantum dots is higher than that of pristine ZnIn2S4.It proved that the ternary sulfide nanocrystals have great potential in the field of photocatalytic hydrogen production in the visible light region.2.The quasi-type-II Zn-AgIn5S8/ZnS core-shell heterostructure photocatalyst were synthesized by in situ growth via a low temperature hydrothermal method.The optical properties of the samples coated with ZnS shell were studied by UV-vis absorption and fluorescence spectra.The ultrathin ZnS shell plays an important role in quasi-type-II Zn-AgIn5S8/ZnS core-shell heterostructure photocatalytic water splitting system,which could reduce surface defects,prolong the carrier lifetime and improve the photogenerated electron-hole pair separation effectively,resulting in improved photocatalytic efficiency and enhanced stability of the catalyst.The results provide an effective guidline for the shell thickness design in future construction of the quasi-typeII core-shell heterostructure photcatalys.3.The Zn-AgIn5S8/g-C3N4 quantum dots/nanosheets composites were prepared by in-situ growth method from the inexpensive two-dimensional g-C3N4 nanosheets.The crystal phase,microstructure,light absorption and photocatalytic water splitting performance of the composite were studied in an effort to determin the optimal proportion of composite materials.The mechanism of Zn-AgIn5S8/g-C3N4 composite photocatalytic hydrogen production were also studied.The photocurrent and impedance analysis show that the construction of heterojunction is beneficial to reduce the recombination of electron-hole pairs,improve the separation of photogenerat io n carriers and enhance the stability of sulfide quantum dots.When the mass ratio of gC3N4 was 10%,the efficiency of Zn-AgIn5S8/g-C3N4 composite hydrogen production was maximum under visible light irradiation,and the hydrogen production efficie nc y was improved by 1.39 times compared with that of pure Zn-AgIn5S8 quantum dots.This work provides a relatively simple method for constructing high-quality quantum dots/nanosheets 0D/2D heterojunctions and has a certain guiding significance for improving the stability of sulfide photocatalysts.4.The effect of MoS2 on the structure,light absorption and reactivity of ZnAgIn5S8/MoS2 composite were studied by in-situ hydrothermal deposition of MoS2 on the surface of Zn-AgIn5S8 quantum dots.The study show that superoxide radicals play a major role in the degradation of RhB.The fluorescence lifetime indicates that the interface promotes the separation and transfer of electron hole pairs,and the photoinduced electron transfer from Zn-AgIn5S8 quantum dots to the MoS2 cocatalyst.By further analysis of PL quenching and lifetime of RhB upon the addition of ZnAgIn5S8/MoS2 composites,it was found that MoS2 has a major influence on the interaction between the dye molecules and photocatalysts,for which a continuo us charge flow mechanism was proposed with the introduction of MoS2.These findings provide an interesting perspective and important guiding significance for the use of the low-cost two dementional MoS2 in photocatalyst design in the future.